(51) Earth Science

The most beautiful gems & minerals you'll ever see!

Minerals and Gems (National Geographic)


Minerals and Gems

When you hear the word minerals, what comes to mind? Do you picture a cereal box advertising extra vitamins and minerals? Do you think of miners spending years searching for a glimpse of a shiny nugget or a brilliant stripe across a rock face? Or the many-faceted beauty of a friend’s diamond ring? Rock found on the Earth’s crust is a solid material created by three main geological processes: magma solidification, sedimentation of rock layers, and metamorphism. As a result, three basic rock types are formed.

* Igneous rock (volcanic or plutonic) is formed by the solidification of molten magma from the mantle.

* Sedimentary rock is formed from the burial, compaction, and lithification of deposited rock debris or surface sediments.

* Metamorphic rock is created when existing rock is chemically or physically modified by intense heat or pressure.

Geologists usually consider rocks to be a jumble of naturally occurring materials, mainly minerals. They can contain a mix of minerals and other organic substances ranging from microscopic mineral grains or organic matter to rough mineral agglomerates. Rocks can range in size from pebbles to mountains.

When people talk about their ‘‘rock collection,’’ they usually mean their ‘‘mineral collection.’’ Although some people collect rocks, mineral collectors are more common. They are the people looking for the ‘‘perfect’’ example of a specific mineral or the ‘‘rarest’’ specimen within a mineral group.

Amateur mineralogists and collectors are a lot like people who show dog breeds, like German Shepherds or English Pointers, to name a few. They get more points for having a specimen that meets the standard characteristics for the rock type and is of a high priority.

People value things that are rare and perfect. Flawless diamonds are much more valuable than those with flecks and flaws.

In fact, people have decorated themselves with shells, pieces of bone, teeth, and pebbles for the past 25,000 years (Paleolithic Period). But at that time most of the stones they chose were soft and brightly colored. Red carnelian and crystals were common choices.

From the time between 3000 and 2500 BC, lapis lazuli from Dadakshan reached Egypt and Sumer (Iraq). China, Greece, and Rome got their gemstones from many of the same regional mines.

Then, as people traveled and traded more, stones were made into family or governmental seals. They had different textures and some were carved.

When rolled on damp clay, an imprint was made that identified a product. Seals were part of a leap in commercial trade. Some stone seals were worn around the neck and considered a status symbol. Kings and rulers had ring seals that were recognized as symbols of identity and power.

Ancient people thought gems and crystals had special powers. In an uncertain world, people wore them for protection. Color was important in the imagery. Gold was related to the Sun, blue to the sea, sky, or heavens, red to blood or the life force, and black for death. Wearing powerful gems was thought to protect the wearer’s health, and bring wealth, luck, and love.

When the mummy of King Tutankhamun (1341BC–1323BC) a Pharaoh of ancient Egypt, was discovered, it was decorated with gold, red carnelian, turquoise, crystals, jasper, obsidian, alabaster, amazonite, jade, and lapis lazuli. These were the amulets of wealth and strength at that time. These stones were worn during life and some placed on the mummy after death as a protection against harm in the afterlife.

Some minerals and gems were thought to be powerful by themselves, while others were thought to wield power through the figures and words written on them.

Minerals and gems were also thought to contain medicinal powers. The early Greeks recorded these claims in medical papers known as lapidaries. The Greek philosopher, Theophrastus (372–287 BC) wrote the oldest surviving book on minerals and gems, called On Stones. He grouped 16 minerals into metals, earths, and stones (gemstones). A natural geologist, he accurately described physical characteristics of color, luster, transparency, hardness, fracture, weight, and medicinal benefits.

Pliny the Elder (AD 23–79) pulled together everything that earlier scholars had written into his 37-volume series, Historia Naturalis. Pliny’s work provided a lot of useful information on sources, mining methods, uses, trade, and gem value.

Since the 1600s, scientists have become even more questioning. The study of minerals and gems has become a part of the study of chemistry, optics, and crystallography.

Minerals are often described by their chemical formulas in order to note the chemical substitutions of one or more atoms. For example, topaz, a prismatic crystal with the formula, Al2SiO4 (F5OH)2, has been found to be as large as 100 kg. It can be colorless, white, gray, yellow, orange, brown, bluish, greenish, purple, or pink.

Gems and minerals are at the heart of the study of geology. Whether in the Earth or found on other planets, minerals tell the story of a planet’s chemical and physical developments. They have specific characteristics with unique physical and chemical properties. This adds to their great variety and makes the study of minerals interesting.

The study of minerals, minerology, is usually focused on the external microscopic study of minerals in polished sections. People who hunt for and collect rough mineral specimens as a hobby are often called ‘‘rock hounds.’’

Mineral Groups and Properties

All minerals belong to a specific chemical group, which represents their affiliation with certain elements or compounds. The chemical structure of minerals is exact, or can vary slightly within limits. They have specific crystalline structures and belong to different groups according to the way the mineral’s atoms are arranged. Elements like gold, silver, and copper are found naturally and considered minerals.

A mineral is a naturally found, inorganic substance with a specific crystalline structure.

Minerals are classified into the following chemical groups: elements, sulfides, oxides, halides, carbonates, nitrates, borates, sulfates, chromates, phosphates, arsenates, vanadates, tungstates, molybdates, and silicates. Some of these chemical groups have subcategories, which may be categorized in some mineral references as separate groups.

Nine Classes of Minerals

Geologists have identified over 3000 minerals. In order to study them more closely, they have divided minerals into nine different groups.. Minerals occur naturally as inorganic solids with a crystalline structure and distinct chemical make up.

Major mineral groups are determined by chemical composition.

Type Chemical structure

1. Elements

2. Sulfides

3. Halides

4. Oxides and hydroxides

5. Nitrates, carbonates, borates

6. Sulfates

7. Chromates, molybdates, tungstates

8. Phosphates, arsenates, vanadates

9. Silicates

Minerals are divided by different groupings

Mineral – Group (element-e, halide-h, oxide-o, silicate-si, sulfide-su, phosphate-p, molybdate-m, borate-b, carbonate-c) – Hardness (Mohs’ scale) – Chemical (Composition)

Antimony – e - 3 – 3.5 – Sb

Arsenic e 3.5 As

Bismuth – e - 2–2.5 Bi

Carbon (diamond and graphite) –e - Graphite 1–2 Diamond 10 - C

Copper – e - 2.5 –3 - Cu

Gold - e - 2.5 –3 - Au

Nickel – iron - e - 4 –5 - Ni,Fe

Platinum - e - 4 –4.5 - Pt

Silver - e 2.5 – 3 Ag

Sulfur e -1.5 – 2.5 - S

Fluorite - h 4 - CaF2

Halite – h - 2.5 - NaCl

Corundum - o - 9 Al2O3 - (ruby, sapphire)

Cuprite - o - 3.5 – 4 - Cu2O

Hematite - o - 5 – 6 - Fe2O3

Albite – si - 6 – 6.5 - NaAlSi3O8

Anorthite – si - 6 – 6.5- CaAl2Si2O8

Beryl - si 7 –8 - Be3Al2(SiO3)6

Dioptase – si – 5 - CuSiO2 (OH) 2

Jadeite - si - 6 – 7 - Na (Al, Feþ3) Si2O6

Labradorite – si - 6 – 6.5 - (Na, Ca) Al1 – 2Si3 – 2O8

Microcline – si - 6 – 6.5 - KAlSi3O8

Olivine – si - 6.5 – 7 - (Mg, Fe) 2SiO4

Orthoclase – si - 6 – 6.5 - KAlSi3O8

Quartz – si - 2.65 - SiO2

Topaz – si – 8 - Al2SiO4 (F, OH) 2

Zircon - si - 7.5 - ZrSiO4

Cinnabar – su - 2 – 2.5 - HgS

Galena – su - 2.5 - PBS

Pyrite - su - 6 – 6.5 - FeS2

Molybdenite – su - 1 – 1.5 - MoS2

Gypsum - su - 2 CaSO4-2 - (H2O)

Lazulite - p - 5.5 – 6 - (Mg, Fe) Al2 (PO4) 2 (OH)

Turquoise – p - 5 – 6 CuAl6 (PO4) 4 (OH)8  4H2O

Wulfenite m - 2.5 – 3 - PbMoO4

Borax b 2 – 2.5 - Na2B4O5 (OH) 4 8H2O

Calcite - c - 3 - CaCO3

Malachite - c - 3.5–4 - Cu2 (CO3) (OH) 2

Rhodochrosite - c - 3.5–4 - MnCO3

Earth minerals are composed of different elements. Oxygen 47%, Silicon 28%, Alumnium 8%, Iron 5%, Calcium 4%, Sodium 3%, Potassium 3%, Magnesium 2%


The elements include more than 100 known minerals. Many of the minerals in this class are made up of only a single element. Geologists sometimes subdivide this group into metal and nonmetal categories. Of all of the elements, 80% are metals. Gold, silver, and copper are examples of metals.

Carbon produces the minerals diamond and graphite, which are nonmetals. Elements like phosphorus and selenium are also nonmetals. For a complete listing of the known chemical elements, scientists use the Periodic Table of Elements. This is a chart that lists all the elements known today, along with a lot of other useful information. Besides the computer, the Periodic Table is probably the most important tool that scientists use.

Geologists use the Periodic Table to figure out the chemical composition of new minerals and to learn possible ways that different elements might bond.

The Periodic Table of Elements lists an element’s symbol (shorthand name, like C for carbon, Al for aluminum), atomic number (equal to the number of protons), atomic weight, and sometimes the atomic energy levels of the element. When a certain element is described, it is written with the atomic number in superscript and the atomic weight in subscript. On a Periodic Table, magnesium, with atomic number 12 and an atomic weight 24.31, is written as:




While the simplest of Periodic Tables show just an element’s atomic number and weight, complete charts give a broader amount of information. To give you an idea of the usefulness of the Periodic Table, the information listed for titanium in most Periodic Tables is shown below.






Atomic Number – 22

Atomic Weight – 47.90

Group – 4

Period – 4

Transition Metal

Electrons per orbital layer – 2, 8, 10, 2

Valence electrons – 1s2 2s2p6 3s2p6d2 4s2

Knowing specifics about elements, like their electron arrangement, allows chemists and other scientists to figure out the bonding possibilities and types of compounds that can be formed with other elements. From this information, the mineral content of new and unknown samples is worked out. This information is also helpful when creating new compounds in the laboratory.


The halides are a group of nonmetals whose main chemical components include chlorine, fluorine, bromine, and iodine. Most halides are very soluble in water. They also form highly ordered molecular structures with a high degree of symmetry. Halite is the most common mineral of this group. It is known to most people as rock salt. Other halites include the minerals, cryolite, atacamite, fluorite, and diabolite.


A group of minerals, made up of one or more metals combined with oxygen, water, or hydroxyl (OH), is known as the oxides (and hydroxides) group. The minerals in this group show a great variety of physical characteristics compared to other more nonchanging groups. Some oxides are hard and others soft. Some have a metallic luster, while some are clear and transparent. Some of the oxide minerals include anatase, corundum, chromite, and magnetite, while hydroxides include manganite, goethite, tungstite, and diaspore.


The silicates encompass the largest mineral group. As the name implies, these minerals have varying amounts of silicon and oxygen. Silicates are often opaque and light weight. Silicate minerals are different from other groups in that they are all formed as tetrahedrons. However, it can be tough to identify individual minerals within the silicates group. A tetrahedron is a chemical structure where a silicon atom is bonded to four oxygen atoms (SiO4). Some representative silicates include albite, andesine, hornblende, microcline, labradorite, sodalite, leucite, and quartz.


The minerals of the sulfide group are often made up of a metal combined with sulfur. They are recognized by their metallic luster. The sulfides are an economically important group of minerals. The extraction of sulfide ores from composite metals is a standard process in industry. Specific ores are known for certain metal extractions, like cinnabar (a major source of mercury), molybdenite (molybdenum, an alloy in steel), pyrite (iron source), and galena (lead, used in piping and pewter).


The sulfate mineral group usually combines one or more metals with the sulfate compound, SO4. Most sulfates are transparent to translucent, light in color, and soft. They usually have low densities. Gypsum, the most plentiful sulfate, is found in evaporite deposits. Common sulfates include anhydrite (CaSO4) and celestine (SrSO4).

Sometimes, sulfates contain substituted groups like chromate, molybdate, or tungstate in place of the sulfate group. Chromates are compounds in which metals combine with chromate (CrO4). The minerals crocoite (PbCrO4), wulfenite (PbMoO4), and scheelite (CaWO4) are all examples of different group replacements that form different minerals. These compounds are usually dense, brittle, and brightly colored.


The mineral group, known as the phosphates, is made up of one or more metals chemically combined with the phosphate compound (PO4). The phosphates are sometimes grouped together with the arsenate, vanadate, tungstate, and molybdate minerals. These minerals have substituted arsenic, tungsten, and molybdenum elements, respectively.

Although geologists list several hundred different types of these minerals, they are not common. Apatite is the most common phosphate mineral. Most minerals in these groups are soft, but their hardnesses can range from 11/2 to 5 or 6 (turquoise). Although brittle, they have well-formed crystals in beautiful colors like lazulite (blue) and vanadinite (red or orange).


This is an easy one. Carbonates are minerals which contain one or more metals bonded with carbon in the compound (CO4). Most pure carbonates are light colored and transparent. All carbonates are soft and brittle. They are usually found as well-formed rhombohedral crystals. Carbonates react with, bubble up, and dissolve easily in hydrochloric acid. Calcite is the most common carbonate. Other colorful carbonate minerals include rhodochrosite (pink to red), smithsonite (blue green), and azurite (deep blue), and malachite (medium to dark green).

Nitrates and borates are often thought of as a subgroup of carbonates. They are formed when metal compounds combine with nitrogen and boron. When metals bond with nitrate, minerals like nitratine, a rare rhombohedral, transparent, often twinned mineral is formed.

When metals bond with borate, minerals like borax, kernite, and ulexite are formed. Most people have seen white borax, but it can also be colorless, gray, greenish, or bluish. Borax forms near hot springs, in ancient inland lakes, and places from which water has evaporated.


Minerals originally from organic sources (plants) are not usually classified as true (pure) minerals. However, some crystalline organic substances look and act like true minerals. These substances, formed primarily from carbon, are called organic minerals. Amber (petrified tree sap) is an example of an organic mineral.

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